According to Hardin, he tried to obtain a lead role in the film Rio Bravo that had been promised to Ricky Nelson. John Wayne reportedly saw him when he visited a film set at Paramount and was impressed by his appearance.[2] Wayne introduced him to Howard Hawks and William T. Orr at Warner Bros. Television; they bargained for his seven-year contract and he moved to Warner Brothers, who changed his stage surname to "Hardin" after the Texas gunfighterJohn Wesley Hardin.[1] He also attended actors' school at Warner Brothers and acquired small parts in various Warner productions.

When Clint Walker walked out on his ABC series Cheyenne in 1958 during a contract dispute with Warner Brothers, Hardin got his big break. Warner bought out his contract from Paramount Studios and installed him into the Cheyenne show as the country cousin "Bronco Layne" to complete the season. Walker and Warner Brothers came to terms after the season ended, but Hardin had made such a big hit on the show that Jack L. Warner gave him his own series, Bronco, under the Cheyenne title. Bronco alternated weeks with Sugarfoot, starring Will Hutchins, and Cheyenne for four years. The series ran from 1958 to 1962. Hardin was soon given other prominent roles for Warner Brothers productions such as Merrill's Marauders, as Doug "Stretch" Fortune in the 1963 spring break film Palm Springs Weekend, The Chapman Report and PT 109.

When his contract expired he left Hollywood to seek opportunity overseas as his series was played all over the world. Like many other American actors, Hardin traveled to Europe where he made several spaghetti westerns although he turned down Sergio Leone's offer to play the lead in A Fistful of Dollars. He also appeared in American financed all star epics such as Battle of the Bulge and Custer of the West. He also was reportedly the first choice to play the starring role in the television series Batman, which went to Adam West, but turned it down because of film commitments overseas.

Hardin did star in a 1967–1968 Australian television series Riptide[3] where he sponsored an Australian motorcycle racing team and a 1970 German television series called On the Trail of Johnny Hilling, Boor and Billy which was immensely successful in West Germany.

Hardin has been married eight times, divorced seven times, and has ten children from five of his marriages. From 1962 to 1966, he was married to the 1961 Miss Universe, German beauty queen Marlene Schmidt, who later involved herself in the movie industry; they had one daughter. As of 2009, Hardin lives with his eighth wife, Caroline, in Huntington Beach, California.[4]

...I think our movie industry needs a overhaul and [should] start standing up for the values we uphold as sacred and wholesome for our nation. We are a nation founded on Christian principles and values. Even if you don’t agree with our system, don't ignore its success, as Christian believers built the greatest nation this world has ever seen.

The fact that we have turned our backs on our godly virtues is the prime reason our nation is headed for a train wreck. How did our Jesus deal with the money-changers? He threw them out of His church. Who let them back into the church? You did. You’re entitled to believe what you like, but men of God seeking religious freedom established this nation and we own them our allegiance even if we don’t believe in their God of Creation. Our currency carries their trademark, “In God We Trust”. Live with it and respect it, for if we don’t, we will reap a whirlwind of disaster like has never been witnessed before in our history. You may not agree with our Founding Fathers but ... our [leaders] are destroying our nation’s freedoms with financial bondage and their no-win wars. We cannot sit back and watch our nation being reduced into financial slavery while your children are being stationed all over the world protecting their globalist assets. ... We will be called on to retake our land for God. I’m referring to the Bible’s prediction of the last days. What if God is right? Call me an alarmists or a Bible thumper, but I am preparing for the worst and praying for a revival. This present collapse of our economy is just a clear picture of the events to come. ...[5]